I've only played with this a little bit myself, but I think you need to position the two models how you want them in the document window, then in the hierarchy editor you make the parts you don't want in the figure "hidden" (turn the "eye" off), and save as a new figure. THen you move parts of the new figure around to create the proper hierarchies and IK chains, then use Create New Figure on it. I think. So far I've had it claim to work, but then saved things that were no-where near what I wanted, so I'm probably missing a step myself!